This invention relates generally to radio receivers and, more particularly, is directed to an AM sterophonic broadcast receiver.
Systems for transmitting and receiving AM stereo signals are known in the art. In one such system, disclosed in U.S. Pat. No. 4,194,088, a double modulation system is used in which a sum signal (L+R) comprised of a left channel stereophonic signal (L) and a right channel stereophonic signal (R) is used to AM-modulate a carrier signal and a difference signal (L-R) is employed to phase-modulate the carrier signal. With such system, an AM stereophonic broadcast receiver is provided and includes a demodulating circuit having an AM detector in a main channel demodulator path to derive the sum signal (L+R) from an IF signal, a sub-channel demodulator path also receiving the IF signal and deriving therefrom the difference signal (L-R), and a matrix circuit for providing left (L) and right (R) channel stereophonic signals at respective outputs thereof in response to the sum signal (L+R) and the difference signal (L-R). In addition, a pilot signal which has been superimposed upon the phase-modulated difference signal (L-R) is separated therefrom in the AM stereo receiver for use in stereophonic display and the like. Alternatively, the difference signal may be used to frequency-modulate the carrier signal in systems of the type used by Belar Electronic Laboratory, Inc.
With such AM stereo receivers, an amplitude limiter is provided in the sub-channel demodulator path and functions to remove amplitude modulations of the intermediate frequency signal such that a substantially constant amplitude signal is produced. This is accomplished by providing a strong limiting characteristic to the amplitude limiter. However, if a noise component is superimposed on the intermediate frequency signal, loud abnormal sounds or noise bursts, for example, scratching and crunching sounds, are produced as a result of the limiting action of the amplitude limiter and which result in substantial deterioration of the reproduced sound. This phenomenom is particularly noticeable if excessive negative modulation occurs. As a result thereof, there occurs deterioration of the left-channel and right-channel information.
In an attempt to resolve the aforementioned problems, it has been proposed to weaken the negative modulation at the transmitter end. However, such proposed method is not preferred because of deterioration in the reproduced sound. Accordingly, a system has been proposed by the applicant herein along with others to obviate the above disadvantages. With such system, the intermediate frequency signal from an intermediate frequency amplifier is of the form: EQU A(1+L+R) cos (.omega.t+L-R)
In the above equation, the (L+R) portion of the amplitude component corresponds to the aforementioned sum signal, the (L-R) portion of the phase-modulation component corresponds to the aforementioned difference signal, .omega. corresponds to the angular frequency of the carrier signal and A corresponds to the level information of the AM stereo signal. As with the aforementioned AM stereo system, the above intermediate frequency signal is amplitude detected to produce the amplitude component A(1+L+R). This signal is supplied through a capacitor to eliminate the DC component of the signal and thereby supply a sum signal A(L+R) to a matrix circuit. The aforementioned amplitude component of the intermediate frequency signal is also supplied to a negative peak limiter where the minimum level thereof is fixed at a predetermined level and the resultant signal is then supplied to one input of a dividing circuit. The dividing circuit divides the intermediate frequency signal by the output from the negative peak limiter so as to remove the amplitude component from the intermediate frequency signal and thereby provide the phase-modulation component cos (.omega.t+L-R) of the intermediate frequency signal. This signal is multiplied by a non-modulation carrier sin .omega.t produced from the intermediate frequency signal and the multiplied output is supplied through a low-pass filter in which the carrier component thereof is removed. The output of the low-pass filter corresponds to the difference signal (L-R) and is supplied to the matrix circuit along with the aforementioned sum signal, the latter circuit functioning to produce the left (L) and right (R) channel stereophonic signals for reproduction.
With this circuit, when the negative modulation is excessive, no noise bursts are produced, whereby noise does not greatly affect the reproduced sound. However, it is to be appreciated that, with this system, the sum signal contains level information A related to the AM stereo signal, whereby the level of the sum signal changes in accordance with changes in the level of the intermediate frequency signal. On the other hand, the difference signal does not contain such level information A and the level of the difference signal thereby does not change with changes in the level of the intermediate frequency signal. As a result, separation between the levels of the sum signal and the difference signal will increase, resulting in deterioration of the reproduced left (L) and right (R) channel stereophonic signals.
There has also been proposed a modification of the latter-mentioned AM stereo system by the applicant herein along with others in which the negative peak limiter is replaced with a capacitor to remove the DC portion of the AM component from the amplitude detector. The output from the capacitor is combined with a fixed DC component which is independent of the level information A. Again, with this system, the same advantages are obtained as with the latter-mentioned system, that is, prevention of noise bursts. Further, although the level of the output of the dividing circuit will change with changes in the level of the intermediate frequency signal, the distortion factor becomes unsatisfactory. In other words, with this system, since the sum signal and difference signal both contain the level information A, the signals supplied to the matrix circuit will both change in accordance with the intermediate frequency signal. However, since the DC component added to the output of the capacitor does not equal the value of the level information A, a complete division operation in the dividing circuit cannot be obtained and an undivided component is mixed with the difference signal to cause distortion.